The invention relates to a fluid friction clutch, especially for a cooling fan.
Fluid friction clutches of this kind are known, for example from U.S. Pat. No. 3,463,282. They comprise in general a rotor rotatable about an axis of rotation and a housing rotatable coaxially in relation to the rotor and containing a reservoir chamber for a viscous fluid or shear fluid and a working chamber. The rotor is arranged in the working chamber and forms, together with wall faces of the housing, at least one shear gap which, when it is filled with viscous fluid, transmits the torque from the rotor to the housing. A temperature-control device controls the circulation of the viscous fluid between the reservoir chamber and the working chamber and comprises a temperature-dependent valve and a pump device which is effective when the rotor and the housing are in relative rotation. The valve temperature-dependently controls the inflow of the viscous fluid from the reservoir chamber into the working chamber, while the pump device conveys the viscous fluid back from the working chamber into the reservoir chamber.
The pump device of conventional fluid friction clutches comprises an annular pump gap into which there extends a scraper held on the housing. The scraper presses the viscous fluid into the pump gap, and the dynamic pressure forces the viscous fluid back through an opening or passage into the reservoir chamber. In the case of the clutch according to U.S. Pat. No. 3,463,282 the pump gap is defined by mutually radially opposite annular faces on the rotor and on the housing.
The fluid friction clutch must fulfil a number of requirements. By reason of its design the shear gap fills with viscous fluid when the clutch is not rotating. If the internal combustion engine is started in the cold condition the clutch is initially engaged, which per se is undesired. The time interval until the subsequent disconnetion of the clutch should be as short as possible, which can be achieved by a high pump power of the pump device. In pump devices with high pump power the scraper gap between the scraper and the opposite annular surface of the pump gap is very narrow, in order to avoid dynamic pressure losses.
On the other hand a comparatively high pump power of the pump device effects a great actuation hysteresis, that is a great temperature difference between the engagement temperature and the disengagement temperature of the clutch. Great values of the actuation hysteresis result especially from the fact that the valve controlling the inflow of the viscous fluid to the working chamber must open relatively wide in order to balance the high pump power of the pump device. Great values of the actuation hysteresis are undesired, since they impair the temperature regulation properties of the clutch.
The invention is directed towards provision of a fluid friction clutch, especially for a cooling fan of an internal combustion engine, which has good cold-start properties and also a low actuation hysteresis at the same time.